Switch to mdBook, add dev docs

[kacperzuk-neti]

On grant milestone evaluation it was pointed out that deployment of our docs is broken.

This PR drops the broken Jekyll based docs and replaces them with mdBook.

Rationale for mdBook instead of Jekyll:

• standard in Rust and substrate environment
• allows testing of code snippets
• Markdown-based, so all docs previously written could be simply copied, nothing was lost here

This PR contains a GitHub Actions workflow that automatically deploys this to gh-pages branch. As such, if this PR is merged, we can just switch the GitHub Pages branch from main to gh-pages in repo configuration to use it on https://liberland.github.io